Cirro-stratus clouds covering the western horizon to a height of 30 degrees or more indicate rain within twelve hours as a rule. This is a still more certain indication if the lower edge of the cloud is wavy. A cirrus patch with streamer edges, which increases in size, indicates snow.
Clouds moving apparently against a surface wind in reality are moving with an upper current of the air. That is, crosswinds are blowing, and cross-winds very commonly precede rain or snow; a departing storm may also clear with crosswinds.
When ye see a cloud rise out of the west, straightway ye say: There cometh a shower; and so it is.—Luke xii, 54.
The greasy, gray clouds which are characteristic of tropical skies during the rainy season are sometimes seen during summer in northern latitudes. They are pretty certain to indicate a heavy downpour.
Greenish-tinted masses of cloud collecting in the southeast indicate heavy rains.
A mackerel sky—twelve hours dry.
Rain from high clouds or from thin clouds does not last long.
If detached clouds increase in size the moisture of the air also is increasing; if they decrease in size and disappear the moisture is decreasing. While the former condition in itself does not indicate an approaching storm, it is instructive in connection with other local indications.
A sky overcast with high clouds does not indicate stormy conditions if the clouds remain high. If the pressure falls and the clouds lower, stormy weather may be expected.
Rapidly increasing cumulus clouds indicate thunder-storms. A thunder-head or high cauliflower top to a cumulus cloud denotes a rapid updraught, which in itself is the beginning of a thunder-storm.
Still and very slowly moving cumulus clouds indicate a continuance of fair weather.
A cloud layer against the side of a mountain range, if rising to a greater height, indicates increasing pressure; if dropping lower, decreasing pressure.
Cirro-stratus together with alto-stra,tus clouds indicate precipitation with a probability of about 90 per cent.—McAdie.
